News Release Number: STScI-2001-29

September 12, 2001: Data from the Space Telescope Science Institute's Digitized Sky Survey has played an important supporting role in helping radio and X-ray astronomers discover an ancient black hole speeding through the Sun's galactic neighborhood. The rogue black hole is devouring a small companion star as the pair travels in an eccentric orbit looping to the outer reaches of our Milky Way galaxy. It is believed that the black hole is the remnant of a massive star that lived out its brief life billions of years ago and later was gravitationally kicked from its home star cluster to wander the Galaxy with its companion.

Q & A: Understanding the Discovery

1.
Which telescopes were used to observe this black hole?

The black hole XTE J1118+480 was first discovered by the Rossi X-Ray satellite on March 29, 2000. Astronomers used the VLBA to observe it in May and July of 2000. The VLBA's great resolving power, or ability to see fine detail, was used to precisely measure the object's movement against the backdrop of more-distant celestial bodies. Important supporting evidence also came from studying optical images made for the Palomar Observatory Sky Survey (POSS) taken 43 years apart. The Digitized Sky Survey scans, combined with data from both the radio and optical images, allowed the astronomers to calculate the object's orbital path around the galactic center. This is a great example of applying multiple tools of modern astronomy  telescopes covering different wavelengths and digital databases  to a single problem.

2.
How did it get into such an orbit?

This is the first time that a black hole's motion through space has been measured. XTE J1118+480 is in an orbit around the Galaxy's center that takes it far from the Galaxy's plane, moving at 300,000 miles per hour (145 kilometers per second) relative to the Earth. There are two possible scenarios that can explain the black hole having this type of orbit. Either it formed in the Galaxy's plane and was somehow kicked out of the plane, or it formed in a globular cluster and was kicked out of the cluster. As massive stars ends their lives by exploding as supernovas, they leave either a neutron star or a black hole as a remnant. Some neutron stars show rapid motion, thought to result from a sideways "kick" during the supernova explosion. This black hole has much more mass  about seven times the mass of our Sun  than any neutron star. It is therefore likely that it was gravitationally ejected from the globular cluster. Simulations of the gravitational interactions in globular clusters have shown that the black holes resulting from the collapse of the most massive stars should eventually be ejected from the cluster.